This invention relates to a method for mechanically and electrically bonding a metal material to a semiconductor material, especially to provide a semiconductor electrical heater. More particularly, the invention relates to a method for laser-welding a metallic tab to a semiconductor heater element for use in electrical smoking articles.
It has been shown that semiconductor material, such as silicon doped with impurities, can be used to form resistive heating elements having a relatively large, preselectable, electrical resistivity. Such heating elements can be used in compact, lightweight heaters that can be repeatedly cycled from room temperature to temperatures in excess of 800.degree. C. without substantial thermal degradation or changes in operating characteristics. Such a heater has been described, for example, in above-incorporated U.S. patent application Ser. No. 07/943,505. The heater described in application Ser. No. 07/943,505 can be used in an electrically heated smoking article, such as that described in commonly-assigned U.S. patent application Ser. No. 07/943,504, filed Sep. 11, 1992, which is incorporated herein by reference in its entirety.
As described in the above-incorporated commonly-assigned applications, electrical power is applied to each heater element through respective ohmic contacts. The ohmic contacts must have a low electrical resistance in order to concentrate the generation of heat in the bulk of the semiconductor material and thereby avoid excess power consumption and thermal degradation of the contact area. Furthermore, the contacts must be mechanically and chemically robust in order to withstand thermal stresses and oxidation caused by repeated heating cycles, and mechanical stresses caused during use by the mechanical contact of tobacco smoking material against the heating elements. Ideally, the contacts should be easy and inexpensive to make.
In order to apply power from a metallic conductor through the ohmic contacts, above-incorporated application Ser. No. 07/943,505 teaches (a) using mechanical clips that clamp against ohmic contact pads, (b) bonding metallic leads to ohmic contacts or (c) providing metallic "tabs" that make direct ohmic contact with the semiconductor heating element.
The present invention relates to an improved method for providing metallic "tabs" that make direct ohmic contact with a semiconductor element. The method can be used in forming a semiconductor electrical heater for use in an electrical smoking article.